This invention relates to a propfan engine having two oppositely rotating fan rotors and, more particularly, to a propfan engine having two oppositely rotating fan rotors which are both driven by a low-pressure turbine via a drive shaft.
Known propfan engines of the above-mentioned type have either been constructed with transmissions transmitting all of the power supplied by the low-pressure turbine to the fan rotors and then branching it off correspondingly or, each of the two fan stages is coupled with its own shaft by means of which the corresponding power can be supplied to each fan stage by a separate turbine.
In both types of propfan engines, the rotational speed of the fan stages is relatively low because the tip speed must be kept within certain limits for noise reasons. For those engines tested having a fan diameter of approximately 2.5 to 3.5 m, the rotational speeds are in the range of between 1,200 r.p.m in the case of large engines, and 1,900 r.p.m. in the case of smaller engines.
German Patent Document DE-OS 37 14 990 discloses a propfan engine having a differential gear in which the rear fan stage is driven by way of the ring gear of a planetary transmission. The front fan stage is driven by way of a planet carrier. This requires relatively high low-pressure turbine rotational speeds, mainly for smaller engines, because the transmission ratio for these transmissions cannot be constructed to be below approximately 4.0 to 4.5 in the case of the same rotational speed but with an opposite rotating direction of the two fan stages.
Although the development of such low-pressure turbines having rotational speeds above 8,000 r.p.m. is aerodynamically possible, it results in considerable mechanical problems. These problems concern the fact that the occurring edge loads on the rotor blades are very high and lead to high tensions in the rotor disks and the rotor blades. Because of the high rotational operating speed, a supercritical low-pressure turbine shaft course must be expected. This critical shaft course requires a difficult vibration-technical coordination of the rotor system. In addition, such transmissions incur high technical costs because of the high performances which must be achieved.
As a result of uncoupling the fan turbine and the low-pressure turbine by means of the transmission, it must be expected that very high axial forces on the thrust bearings of the engines will occur. This has the result of requiring an increase in the engine weight as a support with respect to these forces.
German Patent Document DE-OS 38 12 027 discloses a propfan engine having two fan stages. The propfan engine is driven by two turbine stages via two separate shafts. This arrangement requires a two-stage low-pressure turbine, the rotors of which are arranged either behind one another or in opposite directions inside one another. However, both solutions have the disadvantage of requiring high technical expenditures and result in technical problems with respect to their bearing. This is because they have two shafts or, when taken together with the gas-producer shaft, three shafts.
Another serious problem in the case of known propfan engines is the ability to guide the two shafts through the core engine. The narrow points through which the shafts must pass include the bores in the disks of the first high-pressure compressor stage and of the high-pressure turbine. The size of the disk bores and the diameters of the shafts must be coordinated with one another to allow permissible disk tensions as well as satisfactory behavior of the engine with respect to the shaft dynamics. Satisfying these requirements, which tend to require contrasting solutions, necessarily leads to compromises, i.e., the expectations must be lowered for all requirements.
There is therefore needed a propfan engine which has a technically simple construction and is as low as possible in weight.
The present invention meets these needs by providing a propfan engine in which one of the fan rotors is directly driven and the other fan rotor is driven by the directly driven fan rotor through the use of a reversing transmission.
The present invention has a number of advantages. Only one shaft has to be guided through the core engine. The power passing through the reversing transmission amounts to only half the overall power thus resulting in a considerable reduction of the volume and the weight of the transmission in comparison to the conventional transmission solution.
It is advantageous that the direct drive may take place either for the rear or the front fan rotor since the power absorption of the fan rotors may differ. In this case, the direct drive takes place for the higher-power fan stage which leads to a further reduction of the volume and the weight of the transmission.
In a preferred embodiment of the invention, the reversing transmission is constructed as a planetary transmission. This construction has the advantage of a lower weight and, particularly because of its ring-shaped construction, is suitable to be installed in the propfan engine. In this case, the planet gears are preferably disposed in the drive shaft. In the case of both embodiments, the arrangement of the invention permits a particularly space-saving transmission arrangement under the tight conditions found in an engine.
In another preferred embodiment of the invention, the planet gears are provided with pinions which are stepped in their diameter and of which one pinion mates with a ring gear coupled with the second fan stage, and the other pinion mates with a housing ring gear supported at the housing. In this case, the pinions are preferably mounted on both sides of the planet gears, i.e., they are suspended in their center area. This has the advantage of a uniform loading of the bearings whereby a bearing space as small as possible is achieved.
Preferably, the two pinions are stepped in their diameter in such a manner that the fan rotor which is driven by way of the reversing transmission rotates at an identical and opposite rotational speed as the directly driven fan rotor. It is also possible to select a different pinion ratio to intentionally provide unequal rotational speeds of the two fan rotors if desirable for aerodynamic or other reasons.
Another advantage of the present invention has the planet gears additionally driving a medium-pressure compressor. Such a compressor, which is frequently called a booster, permits a further increase of the pressure ratio and can be driven in a particularly simple manner by means of a third pinion on the planet gears. For this purpose, a compressor ring gear is provided which mates with the third pinion and is connected with the medium-pressure compressor. By selecting an appropriate pinion, an arbitrary ratio can be achieved from the direction of the drive shaft.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.